Preparation of p-cymene from a monocyclic terpene



Patented Oct. 30, 1945 PREPARATION OF p-CYMIENE FROM A MONOCYCLIC TERPENE Washington Hull, Noroton Heights, Com assignor to American Cyanamid Company, New York, N. Y., a corporation of Maine No Drawing. Original application January 9,

1943, Serial No. 471,878. Divided and this application December 11, 1943, Serial No. 513,991

2 Claims. (Cl. 260-688) This invention relates to the catalytic dehydrogenation of terpenes. More particularly, the invention relates to the production of para-cymene by the vapor phase dehydrogenation of such monocyclic terpenes as dipentene, limonene, terpinolene, terpinenes and the like in the presence of a suitable catalyst containing oxides of chromium and manganese. The present invention constitutes a division of the subject matter contained in my copending application Serial No.

has long been considered as a method of obtaining the latter on a commercial scale. However, heating alone has proved to be inadequate to carry out the reaction. As a consequence, considerable work has been done in investigating catalytic methods for promoting the reaction. I

Because ofits adaptability to continuous production on a large scale, catalytic. dehydrogenation in the vapor phase has received particular at- 'tention. Unfortunately, while the process is desirable from a procedural point of view, its prac-' tice has been subject to a number of drawbacks which in the past have prevented its commercial development.

The principal difllculty encountered in utilizing the process has'been in the development of a suitable catalyst. The reaction has been carof such catalysts as fullers-earth. kieselguhr, ac-

tivated carbon and the like. The best results with these catalysts have been'obtained by contacting the terpene with the catalyst at reaction temperatures of IOU-200 C. in the liquid phase and from about 225-350 C. in the vapor phase.

However, these catalysts are too inactive and the conversions obtained are too low to serve as a suitable basis ior commercial development;

When attempts were made to use the higher operating temperatures with these catalysts, the final results were poor. While it might be generally supposed that the conversion to cymene could be improved by raising the reaction temperature, actually with most catalysts the extent to which cracking occurs is found to increase even more rapidly. An examination of the illustrative equation set forth above shows that the reaction is not simple. Not only is it necessary to remove hydrogen from some of the carbon molecules but it must also be added to others. Hence the reaction has sometimes been referred to as involving both dehydrogenation and hydrogenation.

The reaction is further complicated by the possibility of cracking both the original terpenes and the wholly or partially dehydrogenated reaction products. Therefore, itis not particularly surprising to flnd that raising the temperature with the catalysts proposed by the art actually reduced the cymene production whereas the production of such side reaction products as toluene, ethyl toluene, menthane and the like was increased. In some cases considerable isoprene is found. As a result, it was thought that temperatures above about 325 C. could not be safely used.

Nor did attempts to make use of more active types of dehydrogenation catalysts provide an acceptable solution to the diillculty. Metallic catalysts, such as nickel or copper for example, are too active. In order to prevent excessive cracking and tar formation they must be used at such low temperatures in producing para-cymene that the equilibrium conditions are unfavorable. Metallic catalysts of these types as well as the easily reducible metal oxides are particularly troublesome in thatthey tend to crack oi! the isopropyl group.

Many catalysts werefohnd to produce a disproportionate yield of para-menthane. This latter is particularly unfortunate sinceonce the proried out with some degree of success making use 0 duction of para-menthsne in appreciable quantiproducts by increasing the amount of cracking which occurs.

Based' on these experiences the art believed that it was necessary to 'find a catalyst which would not only give a good conversion without a disproportionate yield of para-menthane, but that it must also find a catalyst which would be operative at temperatures low enough to prevent cracking difllculties. One process, developed by research along these lines, is set forth in my U. S. Patent 2,272,711.- In that case it was shown that by the use of a flnely'divided palladium metal catalyst on such surface active supports as activated carbon and alumina good results can be obtained. Excellent conversion of the terpene to a product remarkably free from such cycloaliphatic compounds as para-menthane were obtained using reaction temperatures of about 250-275 C.

These catalysts, while giving excellent results,

are subject to a number of inherent drawbacks from the point of view of commercial development. The catalyst itself is, from an industrial point of view, relatively expensive to prepare. Further, since a number of materials tend to poison the catalyst, it is desirable to feed a mate- .rial of high purity in order to prevent undue shortening of the active life of the metal catalyst. In addition, they can not be readily activated merely by heating in the presence .of air, a procedure which is easily carried out and is desirable in a commercial process.

Where it is ,desirable to produce a product of sesame.

material which is converted to cymene. but also for the amount of material which is lost through cracking and other causes. For comparative purposes therefore', an arbitrary index was used. This index is one onedmndredth of the product of the percent of the feed recovered as condensate and the percent converted to cymene. For example, where 75% of the feed is recovered as condensate containing 80% cymene the index is (75x80) 100 or 60. This enables a comparison "of the results obtained when using different operparticularly high purity, the palladium metal catalysts are highly superior. However, from the point of view of commercial production on a large process using a suitable catalyst. which will give a good cymene yield, low in para-menthane content, and without an undue amount of cracking either of the material fed or of the intermediate products. At the same time, it is highly desirable that the catalyst be one which is relatively cheap ating conditions and different catalysts to be made.

In carrying out the process of the present invention, the monocyclic terpenes which constitute the raw materials may be obtained from any suitable source. Dipentene, which is commercially available in acceptable quantities, was found to give excellent results. tained for example by isomerization of a-pinene, which is in turn obtained from the sulfate turpentine produced as a by-product in the sulfate digestion of coniferous woods for the manufacture of paper pulp. The a-pinene may be isomerized as a separate reaction either in the vapor or liquid phase, and the vapors of the resultant monocyclic terpenes passed directly over the catalyst of the present invention. It is not necessary that the monocyclic terpene be pure. Accordingly, the products obtained by the isomerization of e-pinene for example may be used directly without fractionation.

Nor is the process of the present invention restricted to the use .of a particular apparatus. The materials to be treated must be vaporized and the vapors passed over the catalyst mass in the reaction chamber at a temperature and rate dependent upon the amount of catalytic mass being used. The reaction products may be collected as by condensation and separated into their respective components as by fractional distillation. So long as these functions may be performed, the design and exact structure of the apparatus may be varied also at will. The development work was carried out using both glass and stainless steel reaction vessels. However, any

material which is catalytically'inactive, does not to produce, has a long active life and can be readily reactivated. It is therefore anobject of the present invention to develop such a group of catalysts and aprocess by which it can be utilized.

In general the objects of the present invention are accomplished by carrying out the dehydrogenation in the vapor phase usinga catalyst comprising chromium oxide with or without the addition of one or more auxiliary oxides selected from the group comprising the oxides of man ganese, supported on a non-pyrolyti carrier. Contrary to the teachings'of the prior art it was found that it was not necessary to keep the reaction temperature below about 325 C. Not only is it unnecessary but wholly impractical to do so, 'since the reaction temperatures of 150-200" C. higher than the limits imposed by the prior art were found to give the best results. In this way conversions as high as 90% or better were obtained.

In evaluating the experimental results it is necessary to allow not only for the percent of the important in obtaining satisfactory results.

contaminate the materials and is resistant to intersranular attack by the hydrogen liberated during the reaction may be used.

Two controls were found to be of primary importance in carrying out the reaction successfully. These are the temperature at which the reaction is carried out and the rate at which the vapors are passed over the catalyst. While of the two the temperature is the least critical, both are It was found that the optimum temperature varies with the nature of the feed, the amount of feed, the volume of catalyst and the like. Once the approximate optimum is determined, variations from this optimum of from 25-30" C. in either direction are not particularly objectionable, al-

though the'closer the control the better the re-- sults. Too low a temperature results in a poor production, due to the lowering of the reaction rate. Too high a temperature results in excesssive cracking and tar formation. In general, temperatures of from about 425-500 C. were found to be satisfactory for a rate of feed within the ranges tested. The invention, however, is not meant to be so limited since in some cases temperatures as high as 525 C., or higher proved to be useful.

I The rate of feed was found to have the most It may be obthe desired products. In eneral, a feed rate of from about to 100 cc. er hour per 100 cc. of catalyst was found to give good results within the temperature ranges from 425-500" 0., although the invention is not necessarily to be so limited.

Mixed chromium oxide and manganese oxide catalysts such as those of the present invention have a articular advantage, in addition. to their being relatively inexpensive and easily prepared, in that they can be readily reactivated. Further they exhibit little, if any, decrease in catalytic activity after as many as thirty. cycles. Regeneration is readily carried out by heating the mass while passing air over it. With some of the catalysts. particularly those containing oxides of tin, it is well to flush out the catalyst with hydrogen after the burning off. However, this is'not absolutely necessary since acceptable results may be obtained without so doing.

The method of preparing the catalyst for use pronounced efiect on the Eegree of conversion to inthe process of the present invention may be varied according to circumstances without departing from the scope of the present invention. Preferably it should be done under conditions which promote the lowest apparent density so as to create a larg surface area. In this respect the catalysts of the present invention have an I advantage over those set forth in my copending application Serial No. 471,877 filed January 9, 1943. In that casethe oxide masses themselves were used as the catalyst. In so doing it is found that a large proportion of the catalyst mass was not available for the performing of useful work since itis substantially impossible to bring the reacting vapors into contact with all parts of the surface. The catalysts of the present process, being extended by the use of a non-pyrolytic carrier give a much larger volume of equally eifective catalyst for the same weight of oxides since the surface area of the oxides is more efficiently free flow of vapors. They can withstand highertemperatures without physical damage than can the unsupported oxides, so that quicker and more emcient reactivation can be carried out by burning in air. e

In preparing the catalyst so as to obtain the desireddistribution of the oxides on the carrier and maintain the lowest bulk density any desirable procedure may be used. For example, a solution of the metal or metals in water may be used to impregnate the carrier mass after which it can be dried and the insoluble metal compound reduced to the corresponding oxide in situ. The carrier may be impregnated either by spraying on the carrier mass or by adding the carrier mass tothe solution if the latter is sumcient in volume. Another procedure which can be used to obtain satisfactory results is to impregnate the carrier by using a suspension of the metals in their oxide form. This procedure is ordinarily used where it is diflicultor the facilities are lacking to convert the metal compound to the oxide in place.

The physical form in which the finished catalyst occurs may also vary quite considerably. In some cases a support may be made into granules of a suitable size and the oxides deposited thereon. This is probably the easiest and cheapest method and is therefore preferable. However, in other cases it is difficult to obtain coherent granules of the carrier in suflicient quantity in the proper size range. This difficulty canbe met in any one of several ways. The support may be powdered and made into pellets upon which the oxide mass is deposited for example, or the oxides may be mixed with the powder and the whole made into pellets. I

Where dimculty in pelleting the material is caused either by the lack of self-adhesion or by the pellets being difficult to remove from the mold, a binder and/or a mold lubricant may be found useful. Substantially any combustible, wax-like material may be used for this purpose. Good. results may be obtained using a material such as stearic acid in amounts of 1 to 2%. This material is an excellent mold lubricant and imparts the desired coherence to the particles. It is easily burned out after the pellets are placed in the converter. Where a permanent binder is necessary, water soluble silicates may be used. Pelleted material is particularly useful in that it enables a uniform packing of the reaction vessel. A considerable variation in the nature and properties of the oxide which goes to make up the catalyst may also be made without departing from the scope of the present invention. The chromium oxide which is used in combination with the manganese oxide may be ineither the three. or six valent form. -When used alone the trivalent form appears to give the better results. In admixture, either the three or the six valent form may give the better results depending principally on the makeup of the other oxides presant and operating conditions. In many cases the presence of chromium, as an oxide, in both forms is helpful. In general the chromium oxide should invention, however, is not necessarily so limited.

Useful results ar obtained using proportions ranging from 1-3 mol parts of chromium oxideto about 1-3 parts of the auxiliary metal oxide.

'Replacing all or part of the auxiliarymetal oxide with'an alkaline earth 'metal oxide such as barium or calcium usually has a slightly benencial a'ctin on the production rate but decreases the yield of cymene by increasing the cracking rate. The presence of uncombined alkali metal oxides appears to be undesirable in that it provents the proper migration" of the hydrogen which must occur in ,cymeue formation.

All of the catalysts of the present invention also have a marked advantage in that they may be used without the necessity for a diluent gas. Many dehydrogenationcatalysts, useful for other purposes such for example as metallic nickel, are

so active, at the temperatures at which the equilibrium is favorable to cymene production, that the time of contact must be controlled'by means of a diluent. This is highly undesirable since it complicates both the process and apparatus and adds to the cost of operation. In most cases the catalysts of the present invention give better results in the absence of a diluent than they do when such a gas is used.

A number of non-pyrolytic materials are useful in making up the catalysts of the present invention. A number of these are the some materials which were used as catalysts in the earlier prior art. Notable among these are kieselguhr and aluminum oxide. The activated carbon catalyst used in the prior art is also a good material except that it can not be readily reactivated by burning. Among the more useful supports found were alumina and silica gels, such as the ordinary activated alumina of commerce and broken particles of silica bricks. Closely analogous tothe latter, and also as good in use, are broken particles of burnt clay. The clays, however, should be as free as possible from alkaline metal oxides since these assures lected and washed free from 804- ions on I. suetion illter and then dissolved in 105 cc. of concentrated nitric acid which had been diluted with 200 'cc. of water. The resultant solution 'was sprayed onto 300 cc. of 6 to '10 mesh granules of burnt clay refuse while the mass was heated gently. 120 cc. of the resultant product was packed in an upright column and vaporized dipentene passed thereover at different temperatures and feed rates. Reaction'products were collected by condensation and analyzed. Illustrative results are shown in Table I.

Table I Temp. (av.), C. m Index 51. 6 39. 7 20 M. 3 ll. 2 65. 4 48. 8 15 73. 7 M. 5

Exmnx To a solution of 11.2 gms. of MnCO: in'concentrated hydrochloric acid was added cc. of concentrated nitric acid after which the mixture was ready for use, is relatively cheap and exhibits the l'east tendency among the supports tried to promote cracking. The invention, however. is not meant to be so limited since as pointed out good results were obtained using silica gel granules, broken burnt clay particles, titanium dioxide and kieselguhr as supports.

The invention will be more fully explained in connection with the following examples which are meant to be illustrative only and not by way of limitation.

In the following examples, the reaction products were analyzed to find the degree of conversion by passing dry HCl gas through 25 cc. samples of the condensate in an ice bath at the rate of about 2 bubbles per second for 1% hours.

The ice bath was then removed and the gas bubbledthrough at the same rate for 1 hour at room'temperature. Free HCl was removed by evacuating at 10 mm. of mercury for V hour at room temperature. Approximately 2 gms. of the sample was then treated with cc. of neutral methanol after which 50 cc. of standardized alcoholic KOIIwas added and the mixture refluxed for 2 hours. Since the unconverted terpenes form stable hydrochloride: whereas the desirable products do not, titration of the excess KOH after saponiiication gives a measure of the terpenes remaining in the condensate.

Exams: 1

A solution of 52.6 gms. of CrO; in 500 cc, of

water to which was added 10 cc. of concentrated sulfuric acid was prepared. gas was bubbled through the solution at the rate of about 2 bubbles per minute until the chromium was reduced to the trivalent form. The solution was heated evaporated with stirring to remove Cl: and HCl. The resultant Mn N0a a was added to 380 cc. of a solution of Cr(NO:): prepared according to the .procedure set forth in Example 1, containing 0.0836 gm. of ClaOo per cc. The resultant solution was sprayed onto 300 cc. of heated 6 to 10 mesh burnt clay refuse. cc. of the impregnated and dried material was used in carrying out a number of runs by passing vaporized dipentene thereover as in Example 1. Illustrative results are shown in Table II.

Table II Temp. (sv.), "c. 57:; si m?- Index 10 no.1 40.6 15 62.5 46.0 3) 71.0 66.6 45 m 20 68.1 5a.:

389 gms. of Cr(N0:):'-9H:0 were dissolved in water. To one-half of this solution was added an MmNOa): solution containing 9.5 gms. of Mao: prepared as in Example 2. The combined solution was used to impregnate 300 cc. of-6 to 10 mesh granules of silica gel. 100 cc. of the catslyst was'packed in a vertical reaction column and vaporized dipentene passed thereover at different feed rates and temperatures. Illustrative results obtained by analysis of the condensate areshown I claim: LA process of producing. p-eymenc which comprises the steps of taining acatalyst body at a temperature" I fromabout SIP-600 0., completely vaporizing a monocyclicterpene having a single isopropyl'side chain at a rate of llfrommi-mpartofliquidterpeneperplrtot commcatalyst by volume, passing the completely vaporized terpene over the heated catalyst, whereby a major portion of the terpene is converted to p-cymene, condensing the condensable portion of the reaction vapors and isolating the p-cymene content thereirom; said catalyst body comprising 1-3 mol parts of a substance selected from the group consisting o1 CraOa. CrO: and mixtures of the same, together with from 1 to 3 mol parts of manganese oxide, supported on a carrier having a high surface activity by exhibiting substantially no tendency to crack 08 the isopropyl side chain at an operating temperature.

2.-A process according to claim 1 character- WASHINGTON 

